meson: Warn when git is missing in non-dist builds
8b506034 fixes the issue of version headers
no being available when building out of VCS tarball (without .git directory)
but we were getting unknown (unsupported)
version still. Turns out the version
silently fell back to unknown
when git was not available.
Let's warn the user when this is the case.
Edited by Jan Tojnar